Local area networks (LANs) allow multiple users (or "nodes") to utilize a shared media to transmit and receive digital information. The concept of a LAN is so useful that attempts are being made to extend LAN technology to Digital Subscriber Line (DSL) communication systems. DSL systems use existing telephone lines for high speed data communications. A DSL system essentially encodes digital data as analog signals at very high data rates using special modems. One signaling method used to transmit such analog signals is orthogonal frequency division multiplexing (OFDM), in which analog encoded data bits are transmitted as complex tones in distinct frequency bins.
A number of Digital Subscriber Line (DSL) systems have been proposed. For example, a version known as Asymmetric Digital Subscriber Line (ADSL) provides a system that applies signals over a single twisted-wire pair that supports "plain old telephone service" (POTS) and high-speed duplex (simultaneous two-way) and simplex (from a network to the customer installation) digital services. Part of the proposed standard for ADSL is set forth in the Draft Proposed Revision of ANSI T1. 413-1995--Interface Between Networks and Customer Installation--Asymmetric Digital Subscriber Line (ADSL) Metallic Interface (Sep. 26, 1997), which is hereby incorporated by reference.
In order to permit sharing of a LAN media, a media access control (MAC) protocol must resolve "contention" between the users seeking to access the media simultaneously. In a media such as DSL systems, a variety of possible MAC protocols may be used. For example, each node on the network can synchronize transmissions based on simultaneous detection of silence (i.e., no node is currently transmitting data). However, DSL systems are inherently noisy due to cross-talk in telephone wiring bundles, reflections, etc. Further, the wiring environment within a home may be particularly "hostile", with unterminated telephone extensions of various lengths causing reflections. Accordingly, silence is not easily detectable, and positive identification of idle media may be difficult or impossible to achieve.
Another technique is described in co-pending U.S. patent application No. 09/003,844, entitled "Method and Protocol for a Medium Access Control Layer for Local Area Networks with Multiple-Priority Traffic", filed Jan. 7, 1998 and assigned to the assignee of the present invention. In this technique, each node waits for a certain period of silence, then transmit a single randomly selected tone as an intent-to-transmit signal. All nodes receive the combined intent-to-transmit signals of all other signals. Each station makes a determination as to whether it had transmitted the highest frequency, and if so, begins to transmit data. If two or more stations had sent the same frequency as an intent-to-transmit signal, a collision will occur, indicated by the presence of garbled data on the medium. The colliding nodes stop transmitting data and repeat the process above until no collisions exist. A drawback of this technique is that each transmitting node must "listen" to what it is transmitting to detect a collision, which requires data demodulation. Data demodulation requires a substantial expenditure of processing resources and complex processing.
Accordingly, the invention has determined that it would be useful to have a contention resolution MAC protocol that is simple to implement, provides positive identification of idle media, and does not require data demodulation. The present invention provides a method and system for achieving this end.